The present invention relates to a process for the desulfurization of gypsum such as natural or by-product phosphogypsum. The present process provides for the coproduction of a solid sintered material and a gaseous effluent containing sulfur dioxide, sulfur or mixtures thereof.
Natural phosphate rock is the primary commercial source of phosphorous. One of the most common methods of producing phosphoric acid from the phosphate rock is the acid or wet process. The wet process comprises digesting the phosphate rock with a strong mineral acid, e.g., sulfuric acid, to release phosphoric acid. The solid residue of the wet process is impure calcium sulfate or phosphogypsum. Phosphogypsum has, until recently, been considered a waste product of the wet process having no commercial value and thus great mounds of phosphogypsum have accumulated near and around phosphoric acid plants. These mounds of phosphogypsum pose an environmental problem due to the acidulation of rainwater runoff from the soluble compounds in the phosphogypsum.
One commercially valuable process which converts phosphogypsum into useful products is disclosed in U.S. Pat. No. 4,503,018 issued to Gardner et al. (Gardner) which is incorporated by reference in its entirety. The Gardner process produces sulfur and/or sulfur dioxide from gypsum by thermal decomposition of the gypsum. More specifically, Gardner pelletizes a mixture of fine coal and fine gypsum material and charges the pellets to a travelling grate where the pellets are heated under suitable conditions to produce a gaseous effluent containing sulfur dioxide and/or sulfur. After the pellets have undergone the thermal decomposition, the lime residue may be sold or used in conventional applications. Gardner teaches that other carbonaceous material or reducing materials such as coke, petroleum coke, elemental sulfur, pyrite and other sulfides may be used in place of coal.
While the Gardner process is a viable method of converting phosphogypsum into usable products, those skilled in the art are continuously striving to find methods of improving the efficiency of the Gardner process and provide a more economical means of utilizing the phosphogypsum. Surprisingly, the present inventors have discovered that use of the combination of carbonaceous material and pyrite as a feed mix provides significant and unexpected advantages over use of coal or pyrite alone.